Researchers think a combination of chemicals may be responsible for sea lions developing aggressive cancer.

For the past 14 years, scientists have been searching for why so many California sea lions are dying from metastatic cancer. Data from 1996, collected by Dr. Frances Gulland, the Director of Veterinary Science at the Marine Mammal Center in Sausalito, determined that 18% of the deaths in the sea lions were a result of tumors in the reproductive and urinary tracks.

Dr. Gulland said that "it’s such an aggressive cancer, and it’s so unusual to see such a high prevalence of cancer in a wild population. That suggests that there’s some carcinogen in the ocean that could be affecting these animals."

On average, the Marine Mammal Center sees 15 to 20 California sea lions with cancer a year. Just last month, a meager 200-pound male sea lion was found on Stinson Beach, with paralysis in its genital area and swollen hind flippers — both of which are signs of cancer. After years of studying these California sea lions and their habitat, researchers have come to think that the cause is not one chemical, but several. These conclusions are based off of data that points to a number of industrial contaminants in the ocean, which pose rising concerns to both scientists and civilians.